This application relates to a method for detecting partial fill in an electrochemical test strip, and to a meter, and meter-test strip combination for use in such a method.
Small disposable electrochemical test strips are frequently used in the monitoring of blood glucose by diabetics. Such test strips can also be employed in the detection of other physiological chemicals of interest and substances of abuse. In general, the test strip comprises at least two electrodes and appropriate reagents for the test to be performed, and is manufactured as a single use, disposable element. The test strip is combined with a sample such as blood, saliva or urine before or after insertion in a reusable meter, which contains the mechanisms for detecting and processing an electrochemical signal from the test strip into an indication of the presence/absence or quantity of the analyte determined by the test strip.
It is generally desirable in electrochemical test strips to utilize a small volume sample. One of the challenges that is encountered with small volume test strips is the occurrence of partial fill situations, where the volume of sample introduced to the strip is insufficient, resulting in erroneous readings. Various solutions to the problem of partial fill have been proposed.
In many instances, these solutions to the problem involve the use of additional electrodes. For example, U.S. Pat. No. 4,929,426 discloses the use of an impedance electrode that sample flows over when the analysis chamber is filled, while U.S. Pat. Nos. 5,582,697, 6,212,417, and U.S. 6,299,757 all disclose the use of a third electrode that can be used for fill detection. U.S. Pat. No. 6,743,635 discloses a four electrodes approach, including separate fill detect anode and cathode. U.S. Pat. No. 5,997,817 discloses a test strip with a window through which the sample can be viewed, and a “fill-to-here” line to assess sample sufficiency.
U.S. Pat. No. 6,856,125 discloses measurement of capacitance as a way to determine sample volume. The apparatus includes a sine wave generator to apply an AC signal to a biosensor cell containing a sample, a current-to-voltage converter, a phase shifter, a square wave generator, a synchronous demodulator, and a low pass filter which yields a signal proportional to the effective capacitance across the biosensor cell. This signal is proportional to the volume of the sample.
Because electrochemical test strips are generally disposable and multiple strips may be used by a diabetic in a single day, it is desirable to control the cost of each item. It would therefore be desirable to have a system for confirming the sufficiency of sample volume without significantly adding to the component count in the test strip or the meter, and hence the manufacturing cost of the test strip and meter. It would further be desirable if such a system were automated within the test meter, and did not depend on an observation or judgment made by the user.